The present invention relates to a method of teaching a position, more precisely relates to a method of teaching a position, which can be applied to a finishing machine having a grind stone, especially a machine for removing flashes from a cast product.
A gate and flashes must be removed from a cast product, so many types of machines, which automatically remove gates and flashes of cast products, have been developed.
In one of the conventional automatic machines, a rotary grind stone is located at a predetermined position, and a workpiece (a cast product) is moved, by a driving mechanism including a plurality of linked arms, toward the rotary grind stone, so that the workpiece is made contact with the rotary grind stone so as to remove the gate and flashes from the workpiece.
In the conventional machine, a proper machining track of the workpiece is defined before the removing operation. Namely, the workpiece, which has been set in the machine, is moved so as to input positions of parts of the workpiece, which will be machined, in order, to the machine.
The parts to be machined are inputted or taught by two ways. One is manual teaching, in which the workpiece is manually moved; the other is machine teaching, in which the workpiece is moved by machine. The manual teaching or the machine teaching is selected on the basis of machining conditions.
In the case of the machine teaching, when the part of the workpiece contacts the machining tool, the workpiece is coercively moved unless an operator stops the machine, which moves the workpiece. By the coercive movement of the workpiece, great loads are applied to the workpiece, the machining tool, a holding unit for holding the machining tool, arms for moving the workpiece and motors, so that the members are highly damaged.
Even if the workpiece properly contacts the machining tool, the workpiece is pressed on the machining tool with a prescribed force, so the arms, which hold and move the workpiece, are deformed and backlashes are formed in the arms. By the mechanical deformation and backlashes, an actual position of the work piece is different from a calculated position thereof, which is calculated on the basis of detected data of angular-position sensors, which respectively include encoders and respectively provided to joints between the arms, and sizes of the joints. Therefore, it is impossible to correctly teach the parts to be machined as a machining track.
An object of the present invention is to provide a method of teaching a position, in which greater loads are not applied to a workpiece, a machining tool and a mechanism for moving the workpiece.
Another object of the present invention is to provide a method of correctly teaching a position.
To achieve the object, the method of teaching a position of the present invention has a following structure.
Namely, the method of teaching a position on a machining track in a machine comprising:
a first arm turning in a first horizontal plane;
a first motor for turning the first arm;
a second arm provided to a front end of the first arm, the second arm turning in
a second horizontal plane;
a second motor for turning the second arm;
a holding unit for holding a workpiece, the holding unit provided to a front end of the second arm, the holding unit rotating about a vertical axis;
a third motor for rotating the holding unit; and
a control unit controlling the motors to turn and rotate the first arm, the second arm and the holding unit, the control unit moving the workpiece to positions, which have been stored, so as to make a part of the workpiece contact with a machining tool, the method comprises the steps of:
driving the motors with detecting exciting currents, by the control unit, so as to move the workpiece from a position short of an object position to the object position;
making the workpiece contact with the machining tool, which corresponds to the object position;
temporally turning off the exciting currents of the all motors if at least one of the exciting currents, which respectively run through the motors, is greater than a predetermined value; and
storing a position of the workpiece as the object position if counter forces, which work from the machining tool to the arms and the motors, and frictional forces, which are generated in the arms and the motors, are balanced when the exciting currents are turned off and the workpiece is made contact with the machining tool.
In the method of the present invention, since the exciting currents of the all motors are temporally turned off after the workpiece contacts the machining tool, the workpiece and the arms, which move the workpiece, can be slightly moved backward in a state, in which the workpiece contacts the machining tool. With this action, deformation formed in the arms can be removed or highly reduced. The position of the workpiece can be detected, on the basis of angular positions and length of the arms, in the state, in which the deformation in the arms have been removed or highly reduced, and the position can be inputted or taught as the object position. Unlike the conventional method in which the workpiece is held by deformed arms, the method of the present invention is capable of detecting and teaching the correct position of the workpiece.
When at least one of the exciting currents, which respectively run through the motors, is greater than the predetermined value, the exciting currents of the all motors are temporally turned off. Therefore, applying greater loads to the work piece, the machining tool and the arms can be prevented.
Even if an operator is involved in or pinched between the arms during the teaching operation, the exciting currents of the all motors are turned off when the arm or arms contact the operator. Therefore, the operator can be escaped from the arms by small force, so the teaching operation can be safely executed.